One player's character can attempt to help another's at a skill test. To do this, the helping player rolls a d4 on their skill. If they succeed by rolling at or under their skill, then the helped player gets +1 pip for their skill roll.
However, if the helping player rolls a 4, they must check their skill on a d4 again. If they fail the second check, the helped player immediately suffers a catastrophic failure caused by the ineptitude of the helper.
Once one player has make a skill check to attempt something, no other player can make a skill check to attempt that same thing. Either help out, or accept that you've used your one chance to use the rules to get something done for you, and you'll have to come up with more creative solutions if you fail.
Players with a 5 or a 6 in a skill don't need to roll to help, they automatically give +1 pip in help.
Players with 4 pips in a skill cause a catastrophic failure on double-4's, and otherwise successfully help.
If the helped player's skill moves beyond 5 pips due to help, they roll another d6 for each additional pip beyond 5, and only fail if they receive 6s on all the dice that they roll. 2d6 at 6 pips, 3d6 at 7 pips, etc.
Catastrophic failures are context-dependent and up to the GM. On a "bushcraft" roll this might mean an immediate random encounter roll, on a "language" roll it might mean a hostile misinterpretation. The general rule is that more or less the opposite of what the players wanted to happen, will happen.
Multiple players can help if it makes sense in-game, but a single catastrophic failure means doom for the attempt. Roll all the help rolls together first.
Helping characters must have the same or fewer pips as the helped character.
NPCs, retainers, etc., cannot help or be helped by these rules, even if they have skill points. The exception is henchmen, who can help or be helped only by their corresponding PC.